Switching Power Converters have for many years served as a viable means for electrical energy conversion. Unfortunately, although the semiconductor devices used in these converters are operated in a manner similar to that of switches, undesirable energy dissipation internal to these conventional devices nevertheless occurs during turn-on and turn-off transitions. Such losses are due to the simultaneous existence of voltage across and current through the semiconductor devices during commutation. Because these losses occur at each switch transition, high frequency operation correspondingly yields low power conversion efficiencies.
Since higher switching frequencies generally result in smaller reactive components and improved dynamic performance, mechanisms for minimizing switching losses have long been sought after. For example, in conventional Pulse-Width-Modulated (PWM) switch-mode converters, energy recovery snubbers have been used to "soften" the switching of semiconductor devices. A technique known as "soft-switching" has also been implemented in switching power converters. This conventional technique seeks to eliminate switching losses by altering the switching conditions in such a way that the switch current or switch voltage is zero at the time of commutation.
In this way "Zero-Current-Switching" (ZCS) or "Zero-Voltage-Switching" (ZVS) respectively, is attempted. To implement this switching mechanism, an L-C network is added around the switch so that the switch current or switch voltage may be kept at a constant zero value during switch commutation. Conventional switch-mode converters using this type of soft-switch are known as Quasi-Resonant Converters.
ZCS may be attempted when an inductor is placed in series with the semiconductor switch FIG. 1. Since the energy stored in an inductor cannot change instantaneously, therefor neither can the current through it change instantaneously. If energy resonates between the inductor and the capacitor when the switch is on, then the switch may be opened losslessly (in theory) at a time when the inductor has dumped all of its energy to the capacitor. Once the switch is open, the inductor current remains zero, and the switch can turn on with zero-current through it.
ZVS may be attempted when a capacitor is placed in parallel with the semiconductor switch (FIGS. 2 and 3). Since energy stored in the capacitor cannot change instantaneously, therefor neither can the voltage across it change instantaneously. If energy resonates between the capacitor and the inductor when the switch is off, then the switch may be closed losslessly (in theory) at a time when the capacitor has dumped all of its energy to the inductor. Once the switch is closed, the capacitor is shorted and its voltage remains zero, thus allowing zero-voltage-turn-off for the switch.
Both ZVS and ZCS conventional techniques seek to decrease switching losses and attempt to permit high efficiency operation at higher switching frequencies. However, only ZVS is effective in reducing switching losses at high frequencies. The reason for this is that some loss must take place during turn-on of a ZCS switch. Parasitic capacitance across the semiconductor switch stores energy while the switch is off, and releases stored energy internally when the switch is turned on. For this reason, high frequency operation of such conventional converters, even with the attendant switching losses, is possible only with ZVS converters.
In practice, ZCS techniques have often been used in place of ZVS techniques even though switching losses are not altogether eliminated (e.g. ZCS Boost QRC of FIG. 7). With ZVS converters, the large resonant voltage of the resonant capacitor is imposed across the active switch. In some half and full-bridge converter topologies, this resonant voltage is limited by the clamping action of the input voltage source. In single-ended converter topologies, such as the ZVS Boost QRC of FIG. 8, the voltage is unrestrained and may peak at a value equal to ten times or more the input or output voltage. This peak resonant voltage is also a strong function of the output load resistance or current. Therefor at high voltage and/or high power levels, the voltage stresses impressed upon the active switch are intolerable, thus making ZVS implementation in known single-ended converters impractical.
Several techniques have attempted to reduce these high voltage levels in the hopes of making ZVS a viable technique for high voltage and/or high power applications. One such technique known as ZV Multi-Resonant Switching (ZVS-MR) reduces voltage stresses by adding a second resonant capacitor across the rectifying diode(s) of the power converter as shown in U.S. Pat. Nos. 4,860,184; 4,841,220 and 4,857,822. Two resonant capacitors --one across the active switch(es), and the other across the rectifier diode(s)--share the energy resonating from the resonant inductor. In this way, the peak voltage across the active switch is reduced since the high voltage is divided between the two resonant capacitors. A typical ZVS-MR Boost Converter is shown in FIG. 9. Unfortunately, this technique does not solve the problem, particularly so for off-line applications where input voltages may be as high as several hundred volts.
Another technique incorporating the above mentioned multi-resonant technique has also attempted to decrease voltage stresses on the active switch. This technique uses the above mentioned two capacitor multi-resonant circuit, but includes a voltage clamping mechanism for limiting the active switch voltage. This technique implemented in a Boost converter is shown in FIG. 10. The voltage clamping circuit includes a bulk capacitor and an auxiliary switch. Since this bulk capacitor is large in value relative to the resonant capacitors, the voltage across it may be designed to be approximately constant over a switching cycle. When the voltage on the active switch is equal to that of the bulk capacitor, the auxiliary switch turns on with ZVS and energy flowing from the resonant inductor is routed from the resonant capacitor to the bulk capacitor. The auxiliary switch turns back off once the amount of charge which flowed into the bulk capacitor has flowed back out. In this way no net charge accumulates on the bulk capacitor from one cycle to the next and its voltage remains essentially constant. This technique lowers peak voltage stresses, but circuit complexity is increased, and reliability has decreased since failure of the active voltage clamping circuit causes voltage breakdown in the main active switch and consequent failure of the power supply.
One type of converter which has successfully produced lower voltage stress for all active and passive semiconductor devices is known as the Quasi-Square Wave Converter. This converter modifies the switch-mode single-ended converters by placing a resonant capacitor across the active switch and/or the passive switch (diode), while the filtering inductor is replaced by a small resonant inductor. A diode is added in parallel with the active switch, and an second active switch is sometimes added across the rectifying diode (FIG. 11 ). In doing so, all semiconductor switches operate with ZVS, and their peak voltage is passively limited (by the diodes in the circuit) to whatever voltage sources and sinks are present in the circuit.
For example, in a Boost converter, the input filter inductor is replaced by a resonant inductor, and the voltage stress on the switches is equal to the output voltage. Unfortunately, rms currents in the resonant inductor are high, and continuous conduction of the resonant inductor current is impossible. In effect, the QSW Boost converter is no longer driven by an effective current source, but rather a voltage source. A further disadvantage of such a converter is that converter voltage gain is severely limited. For instance, a variable frequency controlled Boost QSW converter has a minimum gain of 2.
In a quasi-square wave boost converter, the voltage stress of the active switch and passive switch is limited to the output voltage since these switches along with the output filter capacitor form a closed loop. In other words, the sum of the two switch voltages equals the output voltage. The two diodes in the circuit will passively turn-on when the voltage on either switch reaches the output voltage. To conserve the original operation of the Boost converter, the input inductor must be restored to a filter inductor (so that input current may be continuous and nearly constant if desired,) and the resonant inductor must be moved to a new location in the circuit.
For ZVS operation of the switches, the resonant inductor is needed to remove the charge stored within the parasitic capacitances of each switch. By adding an auxiliary switch and diode, the zero-voltage-transition (ZVT) converter circuit of FIG. 12 is shown. This converter achieves ZVS operation for the main power switch S and power rectifier D.sub.R, however the auxiliary switch and diode operate with ZCS. Although the power flow is not directed through these devices, losses are nevertheless unacceptable at higher switching frequencies since the voltage across these devices may be as high as 400 Volts in universal input off-line applications. This lossy switching results in the inability to operate at very high frequencies.
Therefor, single-ended switching power converters possessing exclusively ZVS operated semiconductor devices along with low voltage stresses have not been forthcoming. A ZVS-MR converter with an auxiliary active voltage clamping mechanism is not reliable since the clamping is not passive but active, and added complexity is also required to control the auxiliary clamping switch. Quasi-square wave converters possess an inherent passive voltage clamping mechanism, however the basic operation of these converters have been altered from that of their switch-mode counterparts. Input or output filter inductors have been replaced by small resonant inductors causing high rms discontinuous currents. By relocating the resonant inductor and adding an auxiliary active switch and diode, the ZVT converter provides desired ZVS switching for the power switch and power rectifier, but operates the auxiliary active switch and diode with ZCS to yield unacceptable turn-on losses at high frequencies.